In the pulp and paper industry there are various processes which are carried out with a low consistency of pulp, even under 1%. Such processes include, for example, normal and reverse vortex cleaning. Subsequent to vortex cleaning the pulp is directed to a processing stage, as for example a thickener, or to the head box of a paper or drying machine. In any event, thickening always follows vortex cleaning in the paper making process.
The treatment of fibrous material, especially cellulose and wood fiber material, often takes place, as mentioned above, in low consistency suspension. For example, screening with perforated or slotted screens is carried out with a consistency of 1 to 3%. Subsequent to such screening the fibrous material is, for various reasons, thickened to a higher consistency. Often the consistency is thereby raised to the range of 10 to 15%, by way of example, for storage or bleaching.
Thickening is carried out in accordance with modern techniques by means of different types of disk or drum thickeners and curved filters. In conventional drum and disk thickeners the discharge of liquid, i.e. thickening, is based on so-called "gravity deckers", vacuum filters or pressure filters.
In gravity deckers the thickening is carried out by means of a horizontally mounted drum formed of perforated plate and covered with wire cloth. The pressure difference required for the thickening results from the level difference between pulp in the trough and filtrate in the filtrate chamber. Pulp may be filtered either from the inside of the drum to the outside or from the outside to the inside, which latter direction is the most common. In practice the diameter of the drum may be 4 m of which, for example, 60% is underwater. The maximum pressure difference is thus about 20 kPa. The pressure difference at the bottom dead center is zero, which difference increases to its maximum value toward the surface of the pulp in the trough. As a consequence substantially no thickening takes place on either side close to the bottom dead center. The situation is also similar in that part of the drum which is not underwater. Thus, a considerable part of the drum surfaces of the gravity deckers is inefficiently utilized. The capacity of that part or parts of the drum in efficient use also varies according to the prevailing pressure difference relative to the filtering surface. The specific thickening capacity of gravity deckers varies in accordance with the pulp and the running or operating conditions, but is typically in the range of 400 to 700 l/m.sup.2 /min. These types of thickeners are typically used to prethicken low consistency pulp as, for example, from 0.5% to 1.5-5%.
The filtering surface of the drum is kept clean or open to the flow by moving the surface against the filtrate or by using air to clean it. For example, a paper mill producing 500 tons of 90% consistency pulp requires a filter with a diameter of 4 m and a length of 7 m, the surface area being about 88 m.sup.2 of wire surface, to thicken the pulp from 0.5% to 1.5%.
The thickening method using a curved filter is also based on gravity decker filtering. The suspension to be thickened is pumped onto an inclined filtering surface. The thickening capacity is in practice 3 to 5% and the specific capacity of liquid discharge is about the same as that of drum filters. This arrangement has the advantage of not utilizing any mechanically moving members, but it also has a drawback in that the apparatus is relatively easy to clog because the attainment of efficient cleaning is difficult. Curved filter type thickeners are used in the pulp and paper industry when minor thickening and low pulp capacities are involved.
The above-described conventional pulp thickening apparatus or "thickeners" are characterized in that the thickening is carried out using very small pressure differences in more or less open equipment and only part of the filtering surface is utilized.
The small pressure difference and the only partial use of the filtering surface result in a poor ability to discharge liquid. The open construction and operational principles result in the possible inclusion of air in the pulp and filtrate. Air in the pulp decisively weakens, as is known, the infiltration qualities of the pulp.
In other heretofore known and earlier-used arrangements techniques, different types of vacuum filters are most common. The consistency of pulp in these filters is caused to increase through the removal of water from the pulp through a filter surface such, for example, as through a wire cloth covered by a thick fiber mat. In thickening pulp it is possible, by means of the suction effect on the pulp, to use a maximum pressure difference of about 0.5 bar because a stronger vacuum would, undesirably, cause the filtrate to boil.
The pressure difference required for filtering in vacuum and disk thickeners is achieved by a suction leg. Such thickeners differ from gravity deckers in that a pulp layer is formed therein. Thus, subsequent to thickening the consistency of the pulp is 8 to 14%; the capacity of a vacuum or disk thickener is accordingly about the same as that of a gravity decker. The difference between them is that the pulp web, in a vacuum or disk thickener, is formed by suction on the filtering surface through the pulp suspension when that surface is underwater. The filtrate is removed from the web formed on that part of the drum, when the same has risen above the surface of the suspension, so as to achieve the above-noted discharge consistency of 8 to 14%. It is clear that when forming a fiber mat on the filtering surface, the discharge of liquid through the pulp layer substantially slows due to the appreciable flow resistance of the filter web.
It is not advantageous to use this type of thickener for the purpose of prethickening, although it may be employed when the required discharge consistency is relatively high. The specific thickening capacity varies in accordance with the quality of the pulp and the volumetric flow conditions, e.g. in the range of 50 to 300 l/m.sup.2 /min. When employing the above-described apparatus two vacuum filters of that size would be necessary if seeking to achieve a thickened consistency of 10%. The advantage of utilizing a disk filter, rather than a vacuum drum filter, is that more filtering surface can be provided in the same device volume.
A pressure filter differs from a vacuum drum filter in that, in the former, the filtering pressure difference is generated by pressure rather than the suction or vacuum effect of the latter.
The problem with these and many other types of prior art thickeners is their tendency to clog. Where, for example, the pressurized suspension to be thickened is fed to a thickener, the pressure difference is in principle unlimited; in laboratory tests of such an arrangement this type of thickener became clogged by sulfate pulp in ten seconds, after which it had to be cleaned.
Several methods are known to prevent such clogging of or to loosen the web from the filter. For example, in the disclosures of Finnish Patent No. 41712 and U.S. Pat. No. 3,455,821 it is intended to clean the filter surface by vibration. However, the damping ability of the gaseous and fibrous paper pulp prevents the vibrations from having the intended cleaning effect.
Another cleaning method is shown in Finnish Patent No. 68005, according to which cleaning of the disk filter is carried out using compressed air. At a certain stage of the disk sector circulation, compressed air is directed to the inside of the disk sector whereby the air blast loosens the filtered pulp from outside of the sector.
The object of the present invention is to avoid or minimize the drawbacks and deficiencies of the above-mentioned prior art solutions and to create a new method and apparatus for the continuous thickening of 0.5 to 20% consistency pulp, without having to remove liquid through a thick fiber mat format uncontrollably on the filter surface due to a pressure difference and, therefore, also to remove fine impurities in the liquid. The filter plates are theoretically able to remove considerably greater amounts of liquid than in practice because the thickened pulp which collects on the surface of the filter plate effectively prevents the discharge of greater amounts of liquid. Thus, it is possible to considerably increase the filtering and separating capacity if the formation of a thick fiber mat on the surface of the filter plate can be controlled.
A method and apparatus for solving this problem by enabling substantially total fluidization of the pulp flow to be thickened is disclosed in Finnish Patent Application No. 781789 (Gullichsen). The apparatus provides a cylindrical conduit having a perforated wall disposed about a centrally-located rotor. The rotating rotor fluidizes the suspension whereby the fibers of the suspension are separated in the suspension and water can be filtered through the filter surface. As the suspension is totally fluidized a fiber mat is unable to form or collect on the filter surface and thereby plug the openings of the filter surface.
However, a tremendous amount of energy is required in the method and apparatus of this Finnish application of Gullichsen needed to fluidize the pulp flowing through the conduit during the time necessary to separate the liquid. The amount of energy needed thus required can be compared as follows to the energy needed when using the apparatus of the present invention. We may for this purpose assume a situation in which pulp of 10% consistency is dewatered to a consistency of 20%. Gullichsen must fluidize all of the suspension within the filtering chamber, whereby the required energy is E20 kW/mass ton and the rotational speed of the rotor is n20 rpm. E20 is the energy necessary to fluidize pulp at a consistency of 20%; n20 is the rotational speed of the rotor necessary to create shear stresses of sufficient magnitude to fluidize pulp at a 20% consistency.